1. Field of the Invention
The present invention relates to electrical power distribution equipment and more particularly to electrical transformers and switching means therefor.
2. Description of the Prior Art
In some electrical power distribution systems it is desirable to provide a plurality of selectable, often incrementally different, voltage outputs from system transformers. A range of such selectable voltage outputs from a single transformer may be achieved through the use of transformers having a number of isolated multiple tap primary and/or secondary windings interconnected to appropriate switching mechanisms. Commonly used in such applications are bridging tap changers and series-parallel-series (S-P-S) switches.
Bridging tap changers may take the form of several stationary electrical contacts arranged in an arcuate array with a movable contact mounted on an insulating rotor. Rotation of the rotor brings the movable contact into bridging contact with any selected pair of adjacent stationary contacts. Bridging tap changers may be connected to provide selectable voltage outputs by interconnecting the ends and/or taps of transformer windings so as to bypass any or all selected portions (turn groups) of windings.
Two-position series-parallel (S-P) switches and multiple position S-P-S switches are used to connect multiple transformer windings, some of which may be tapped, into various series-parallel-series configurations as well as full series or full parallel configurations. S-P and S-P-S switches characteristically are ganged switch pairs where each switch of each pair has a common terminal.
Bridging tap changers having six stationary contacts (five positions) or eight stationary contacts (seven positions) are the most commonly used in the industry. For this reason, five and seven-position bridging tap changers are easily obtainable as “off-the-shelf” items and are relatively inexpensive. Bridging tap changers with a greater number of positions are usually made to order and therefore are more expensive and have longer delivery times.
It is at times expedient to interconnect bridging tap changers in a manner to provide a common terminal. Such a configuration is achieved in the prior art by “jumper wiring” every other stationary contact of a bridging tap changer in common. Thus, the rotary contact becomes, in effect, the common terminal since in every position of the changer it is in contact with one of the “jumped” stationary contacts. When so wired, a five-position bridging tap changer becomes, in effect, a three-position device and a seven-position bridging tap changer becomes a four-position device, each with a common terminal.
In constructing tap changing selectable output transformers it is desirable to provide a wide range of output voltages available in discrete, relatively small voltage steps. Among the design factors to be considered are cost and ready availability of material or parts, such as switches. Also to be considered, are the winding losses and in particular the winding losses of the highest loss configuration relative to the losses of the lowest loss configuration.